1. Field of the Invention
The present invention relates to fairness control for fair use of the bands of data traffic among stations in a ring-shaped packet communication network such as a RPR (resilient packet ring), etc.
2. Description of the Related Art
An efficient and quick-acting fairness control is requested when traffic congestion takes place in a conventional RPR network (for example, refer to the following patent documents 1 and 2). The RPR and fairness control are standardized by IEEE (Institute of Electrical and Electronic Engineers) 802.17.
Patent Document 1: Japanese Published Patent Application No. 2004-289799
Patent Document 2: Japanese Published Patent Application No. 2004-312672
FIG. 1A shows an example of an RPR network for performing fairness control in a single choke system. The RPR is structured by stations A through J connected in a ring-shaped form. Each station has an add queue 1801, and accumulates a frame passing a congestion point P1 in the add queue 1801. The accumulation time is determined by the fair rate announced in the fairness frame. Under such control, fairness can be realized among the stations.
In this system, a fairness frame is transmitted from each station to only an adjacent upstream station in the direction inverse to the data traffic direction (ring direction). The upstream station determines a fair rate to be announced to a station upper by one stage based on the fair rate of the upstream station and the fair rate announced by a downstream station. For example, only the fair rate at the heaviest congestion point is announced to the station upper by one stage.
FIG. 1B shows an example of an RPR network for performing fairness control in a multichoke system. Each station has add queues 1901 of the number of stations on the RPR, and controls each add queue according to fair rate for each destination station. Thus, the fairness among the stations and effective use of bands can be realized.
In this system, a fairness frame is broadcast from each station to all other stations. Therefore, each station is informed of the fair rate of all stations on the RPR. Each station calculates the possible transmission rate to each of the other stations, and transmits a frame from each add queue based on the calculated value.
However, there is the following problem with the above-mentioned conventional fairness control.
Since there is only one type of add queue in the single choke system, the rate of the frame passing a congestion point is decreased when congestion occurs on the RPR. Therefore, the rate of a frame not passing the congestion point (not related to congestion) is also decreased. Therefore, the bands cannot be effectively utilized. This phenomenon is referred to as a head of line blocking.
For example, in the example shown in FIG. 1A, by a frame addressed to the station F accumulated in the add queue 1801, all frames including those addressed to the subsequent stations D, B, and C are accumulated. In this case, since the frames addressed to the stations D, B, and C do not pass the congestion point P1, they are not related to congestion originally.
In the multichoke system, since each station has add queues of the number of all stations on the RPR, and realizes fairness control by dividing the RPR, the configuration of each station is large and the controlling operation is considerably complicated. Therefore, it is not a practical control method.